1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus such as a copier, a printer, a facsimile, a plotter, or a multifunction printer provided with at least one of these, which includes the optical scanning device.
2. Description of the Related Art
A deflecting unit using silicon micromachining has been studied in recent years, and there is proposed a system in which an oscillating mirror and a torsion bar rotatably supporting the oscillating mirror are integrally formed on a Si substrate, as disclosed in Japanese Patent No. 2924200 and Japanese Patent No. 3011144.
Japanese Patent No. 3445691 and Japanese Patent No. 3543473 also disclose examples of arranging an oscillating mirror instead of a polygon mirror. According to this system, there are advantages that a size of a mirror surface can be made small and the system has low noise and low power consumption although a high-speed operation is possible because of reciprocating oscillation performed by using resonance. There are another advantages that because this system has low vibration and almost no heat generation, a housing for an optical scanning device can be made thin-walled and image quality is hardly affected by this system even if a high-grade resin molding material such as a material with a high combination ratio of glass fiber is not used.
Moreover, Japanese Patent Application Laid-open No. 2007-233235 discloses an example of applying an oscillating mirror to an optical scanning device of a tandem color type. This example describes that by setting a scanning frequency fd according to a resonance frequency of the oscillating mirror and adjusting a beam spot interval p according to the set scanning frequency, it is possible to form a high-quality image without uneven density, color shift, and color error.
In addition, Japanese Patent Application Laid-open No. 2001-281575 discloses an example of an optical scanning device provided with a separation unit that is disposed in the downstream of an optical deflector (polygon scanner). In the example, a plurality of light beams are separated into an array direction of photosensitive drums by the disposed separation unit after being deflected by the deflector and passing through a scanning lens.
As explained above, by using the oscillating mirror, in addition to the advantages of low noise, low power consumption, and low heat generation, there are another advantages that because of a flat configuration in which the oscillating mirror and the torsion bar rotatably supporting the oscillating mirror are formed integrally, the thickness of the oscillating mirror in its normal direction can be made thin as compared with the polygon scanner that has predetermined dimensions (radius of an inscribed circle) from its rotation axis to a mirror surface and that is provided with a rotor in order to ensure inertia.
However, as disclosed in Patent document 5, similarly to the system of using the conventional polygon scanner, the conventional technology employs a system that the rotation axis of the oscillating mirror is arranged so as to be perpendicular to a plane where scanning positions of the photosensitive drums are arranged. Therefore, advantages that the thickness of the oscillating mirror in the normal direction is thin cannot be made full use of.
Moreover, in order to cause the light beams corresponding to colors to obliquely enter the oscillating mirror at angles different from each other in a direction of a rotation axis of the oscillating mirror i.e. a sub-scanning direction, light source units need to be separately disposed in the sub-scanning direction in association with respective inclinations of output optical axes. Therefore, the thickness of the optical scanning device is increased in the direction perpendicular to the plane where the scanning positions of the photosensitive drums are arranged, and this configuration is thereby disadvantageous to lower the height of the device body.
Furthermore, there has been conventionally known an optical scanning device in which a light flux is deflected by a deflecting unit such as an optical deflector and the deflected light flux is imaged as a fine spot beam on a surface to be scanned, and the spot beam is scanned at a constant speed on the surface to be scanned along a main scanning direction. This optical scanning device is applied to a latent image writing unit or the like of an image forming apparatus such as a laser beam printer, a laser beam plotter, a facsimile, and a digital copier. The optical scanning device is configured in such a manner that a laser beam emitted from, for example, a laser light source is deflectively reflected by the optical deflector, so that the laser beam is scanned over the surface to be scanned of an image carrier or the like, and at the same time, by causing the laser beam to be modulated in intensity (e.g., on and off) according to an image signal, an image is written to the surface to be scanned.
Japanese Patent Application Laid-open No. 2001-281575 discloses the optical scanning device provided with the separation unit disposed in the downstream of the optical deflector. By disposing the separation unit and appropriately setting an arrangement position of bending mirrors, it is enabled to reduce a distance from the separation unit to a cylindrical mirror as a most downstream optical element of the optical scanning device.
Japanese Patent Application Laid-open No. 2008-102487 discloses a technology for an optical scanning device capable of forming a high-quality image by adjusting an incident position of a light beam on the optical deflection surface in the main scanning direction and making beam spot diameters uniform over the entire surface to be scanned.
In the abovementioned conventional technologies, however, firstly, a reflection direction of each light flux at the separation unit is set to a rotation axis direction of the polygon mirror. In this case, there is less space capable of accommodating an optical element on a side opposite to the polygon mirror, and the size of the optical scanning device is upsized in consideration of the size of actual bending mirrors and the space for a mechanism for adjusting these.
Secondly, an imaging unit needs to be provided between the optical deflector and the separation unit, which makes it difficult to reduce the distance from the optical deflector to the separation unit, and thus, there is a limit on miniaturization of the optical scanning device.
There is space, around the optical deflector, that can be secured without thickening the size of the entire optical scanning device (see a configuration of an optical scanning device 2000 in FIG. 30). Therefore, there is possibility to provide an optical scanning device suitable for a smaller size thereof. However, in the conventional optical scanning device of a one-side scanning system in which a plurality of light fluxes incident on a light deflector 2001 are deflected on the same surface of the optical deflector for scanning, a separation direction for the light fluxes is the rotation axis direction of the optical deflector, and thus, there is not much space on the side of the surface to be scanned in a writing housing, which causes a size (t′) to be larger. In addition, when a reflection angle at a separation mirror is large, wavefront aberration of the light fluxes may be degraded because of low surface accuracy of the separation mirror.